This K23 application will establish the candidate's independent research career in patient-oriented research in adolescent obesity treatment. The candidate, Brian E. Saelens, Ph.D., is a recently appointed Assistant Professor of Pediatrics at Children's Hospital Medical Center (CHMC) in Cincinnati. This K23 proposal combines mentoring in biological and behavioral factors that influence adolescent obesity. The proposed career development plan will provide the candidate necessary training in body fat distribution measurement, weight-affecting hormones, weight-related behaviors, and pubertal maturation. This training will lead to the development of more efficacious intervention for adolescent weight control, specifically to target the reduction of intra-abdominal fat mass accumulation, as this type of fat accretion is most highly related to the negative health consequences of obesity. The candidate's career plan capitalizes on the biologic obesity-related resources and expertise at CHMC and the University of Cincinnati to augment prior and planned training in the behavioral aspects of pediatric obesity. The proposed research plan consists of a targeted prospective (Study 1) and pilot intervention study (Study 2). Study 1 aims include examining the time course of total body fat and intra-abdominal fat accretion through early puberty among already overweight youth. Study 1 will identify specific periods of relatively high and low intra-abdominal fat accretion. This prospective study also examines potential hormonal and behavioral precursors of changing body fat distribution. Based on Study 1 findings, Study 2 will investigate the differential impact of providing similar behavioral weight control intervention to overweight youth at different time periods of intra-abdominal fat mass accumulation. Intervention efficacy will be examined among overweight youth either 1) timing-matched by intervention during high intra-abdominal fat accumulation or 2) timing-mismatched by intervention during low intra-abdominal fat accumulation. Study 2 will also examine baseline hormonal, body composition, and behavioral correlates of weight control treatment success. These studies will provide exceptional training in the biological aspects of obesity, while also having the potential to begin identifying the timing of behavioral intervention that will optimize health benefit of behavioral weight control programs for the increasing population of overweight youth. The combined skill set and experience garnered from the candidate's career development and research plan will provide the necessary early career support for the candidate to establish a successful independent research career in the important area of pediatric obesity.